During operation, a conventional vented tumble dryer draws air from the surrounding area, heats it, and directs it into the drum of the dryer. The dryer then exhausts the air and retained water vapor through a duct to the outside. As shown in FIGS. 1-3, a known vented dryer 10 generally includes a rotatable drum 12; an air supply duct 14 which introduces air from within the dryer housing or cabinet 16 into the drum 12; a heater 26 supplied at a heater tube portion of the air supply duct 14, which heats the air introduced into the air supply duct 14; and an air exhaust duct 18 to exhaust hot air and water vapor from the dryer, typically to a duct that exhausts the air to the outside of the house or other building in which the dryer is located. A fan or blower 20 is provided downstream of the drum 12 for drawing the air through the system and out the exhaust duct 18. A filter 22 for collecting lint and other debris in the air is placed between the drum 12 and the exhaust duct 18. In such a vented tumble dryer, the sole heat source is the heater 26 upstream of the drum 12. The only heat recovery that takes place is a slight warming of the air drawn into the cabinet 16 before it is drawn into heater 26, by virtue of the heat in the cabinet 16 generated by continued operation of the dryer 10.
Energy efficiency is an important aspect of a dryer, and improved heat recovery offers a valuable tool to improve overall energy efficiency. Some dryer system proposals use partially recirculated air in addition to the conventional heater to improve energy efficiency. These systems mix a portion of the exhaust air with the air being introduced into the drum. See, e.g., U.S. 2010/0146811. The warm, moisture laden exhaust air holds the potential to absorb additional molecules of water when recirculated through the dryer, and thus the heat energy of that air can be reutilized to improve operating efficiency.
However, maintaining the proper amount of recirculated air is important. If too much exhaust air enters the recirculation system, efficiency may decrease. Additionally, warm, moist recirculated air can escape into the dryer cabinet and potentially create condensation internal to the dryer unit, resulting in corrosion and other damage to the components. Some proposed recirculation systems control the amount of recirculated air flow by actively regulating and modulating flaps, dampers, baffles, and the like with, for example, central processing units, sensors, and manually adjustable devices. See, e.g., U.S. Pat. Nos. 5,315,765 and 7,434,333. Such systems can add substantial complexity and cost.
Another concern with using recirculated air is the potential fire hazard caused by lint and other debris that may remain in the recirculated air and be recirculated through the heater. Although most dryers have a standard lint filter, e.g., filter 22 of the dryer 10 shown in FIGS. 1-3, some lint may inevitably remain in the exhaust air flow. Recirculating a portion of this exhaust air back toward the heater poses the risk that accumulated lint may ignite in the heater and be carried into the drum. Thus, some recirculation system proposals include a secondary filter, positioned in the recirculation duct. See, e.g., U.S. 2010/0146811. Some proposed secondary lint filters are cleanable. For example, U.S. 2010/0146811 describes the use of internal scrapers, rinsing agents, rinsing liquids, and other methods of internally cleaning the secondary filter.
Energy efficiency may also be improved with various other methods of heat transfer used in combination with the recirculation system. For example, some laundry dryer proposals aim to improve heat energy transfer by utilizing a heat exchanger to transfer heat from the warm air exiting the exhaust air duct to the cooler air entering the supply air duct. See, e.g., U.S. Pat. No. 5,315,765.
However, prior proposals of dryers with air recirculation systems, or a combination of air recirculation and heat transfer, do not adequately address the practical problems of control, integration, and expense that can impede a successful implementation of these heat recovery techniques. There remains a need for an effective system that may fit and successfully operate within a known dryer design with little modification to existing structure. It would be highly advantageous to be able to provide an easily integrated recirculated air system for a dryer that can direct at least a portion of warm, moist exhaust air back toward the dryer supply duct, heater, and drum, to thereby effectively improve overall dryer efficiency. It would likewise be advantageous to provide such an easily integrated system further making effective utilization of air-to-air heat exchange, to further improve efficiency.